Cyanobacterial harmful algal blooms (cyanoHABs, also known as blue-green algae blooms) are a global problem, fouling water with surface scums and toxins. CyanoHABs are caused by excess nutrient loading (phosphorus and nitrogen) from fertilizer and/or wastewater. Historically, research and management efforts have focused on reducing phosphorus loads into cyanobacteria-impacted lakes. This approach has been successful in some lakes, but other cases show that the resulting nutrient loads can be enriched with nitrogen relative to phosphorus (e.g., Lake Erie, USA, and Lake Taihu, China), which can change the cyanoHAB community and result in blooms that are more toxic. Even when external nitrogen inputs are low, such as during summer drought, dead and dying microbes and plankton are decomposed by bacteria, consuming oxygen and releasing nutrients back into the water, available to be used again by cyanoHABs. Organisms in the lake also prey on other organisms, and like humans, they excrete waste products (pee and poop). These ‘wastes’ include urea and ammonium (NH4+), or other organic material, which is readily converted back to NH4+. Ammonium enhances cyanoHABs, and this process of internal NH4+ recycling represents a vicious cycle of nutrient loading, biomass production, decomposition, nutrient recycling, biomass and toxin production, and so on.

The rate at which recycling processes provide NH4+ for cyanoHABs is much more important to understand and quantify than single, ‘snapshot’ NH4+ concentration measurements, which are limited in space and time. These recycling processes become especially critical when nitrogen concentrations are low. At the University of Aarhus Lake Mesocosm Warming Experiment (LMWE) facility in Denmark, some mesocosms have been enriched with both nitrogen and phosphorus, at three different temperatures to reflect IPCC warming scenarios, since August 2003. The nutrient loading to mesocosms with high nutrient inputs was adjusted in June 2018 by stopping nitrogen inputs, with the aim to provide additional evidence of the consequences of targeting a single nutrient for management. While historical management approaches have focused on reducing phosphorus, modern efforts have evolved to include nitrogen loading reductions in these cyanoHAB-impacted systems. The LWME experiment will last one year, with a return to historical nutrient loading levels in June 2019.

The overall objective of the NCyCLEMe project is to evaluate internal nitrogen dynamics within mesocosms with low and high nutrient loading at two different temperature regimes based on climate warming scenarios, before and after the change in nitrogen loading. We conducted 31 separate suites of incubations from selected mesocosms between 6 Jun 2018, before stopping the nitrogen loads, and 7 Aug 2018, after the nitrogen loading change. With these incubations, we measured water column NH4+ recycling and uptake rates in the water columns of selected mesocosms. From these data, we will evaluate the effects of nutrient loading alterations and temperature on the capacity of simulated lakes to recycle nitrogen, which can then be used to support additional growth and toxin production by cyanoHABs.Globally, future lake management efforts to reduce toxic cyanoHABs must consider these nitrogen recycling rates. Data from the NCyCLMe project will help inform those decisions as the climate continues to warm and nutrient management efforts intensify.

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